Method and device for optimally altering stimulation energy to maintain capture of cardiac tissue

ABSTRACT

An improved system and method for performing automatic capture/threshold detection in an implantable cardiac stimulation device or any device capable of stimulating some organ or tissue in the body. Prior art systems determine the stimulation threshold of the cardiac tissue by detecting an evoked response to a fixed duration stimulation pulse and then increasing the amplitude of the stimulation pulse by a predetermined safety margin value. Such systems are inherently based upon a belief that the chronaxie of the strength-duration curve for a particular patient is essentially fixed and that the rheobase is variable. While this may be true at some times during the patient&#39;s life, e.g., during the acute phase after lead implantation absent drug effects, it is reported that drugs alone may alter the chronaxie and it is believed that other factors may also affect the chronaxie either alone or in combination with the rheobase.

This application claims the benefit of U.S. Provisional Application No.60/204,317, filed May 15, 2000.

FIELD OF THE INVENTION

The present invention is generally directed to an implantable medicaldevice, e.g., a cardiac stimulation device, and is particularly directedto an automatic capture/threshold pacing method for use in such adevice.

BACKGROUND OF THE INVENTION

Implantable cardiac stimulation devices are well known in the art. Theyinclude implantable pacemakers which provide stimulation pulses to causea heart, which would normally beat too slowly or at an irregular rate,to beat at a controlled normal rate. They also include defibrillatorswhich detect when the atria and/or the ventricles of the heart are infibrillation or a pathologic rapid organized rhythm and applycardioverting or defibrillating electrical energy to the heart torestore the heart to a normal rhythm. Implantable cardiac stimulationdevices may also include the combined functions of a pacemaker and adefibrillator.

As is well known, implantable cardiac stimulation devices sense cardiacactivity for monitoring the cardiac condition of the patient in whichthe device is implanted. By sensing the cardiac activity of the patient,the device is able to provide cardiac stimulation pulses when they areneeded and inhibit the delivery of cardiac stimulation pulses at othertimes. This inhibition accomplishes two primary functions. Firstly, whenthe heart is intrinsically stimulated, its hemodynamics are generallyimproved. Secondly, inhibiting the delivery of a cardiac stimulationpulse reduces the battery current drain on that cycle and extends thelife of the battery which powers and is located within the implantablecardiac stimulation device. Extending the battery life will thereforedelay the need to explant and replace the cardiac stimulation device dueto an expended battery. Generally, the circuitry used in implantablecardiac stimulation devices have been significantly improved since theirintroduction such that the major limitation of the battery life isprimarily the number and amplitude of the pulses being delivered to apatient's heart. Accordingly, it is preferable to minimize the number ofpulses delivered by using this inhibition function and to minimize theamplitude of the pulses where this is clinically appropriate.

It is well known that the amplitude of a pulse that will reliablystimulate a patient's heart, i.e., its threshold value, will change overtime after implantation and will vary with the patient's activity leveland other physiological factors. To accommodate for these changes,pacemakers may be programmed manually by a medical practitioner todeliver a pulse at an amplitude well above an observed threshold value.To avoid wasting battery energy, the capability was developed toautomatically adjust the pulse amplitude to accommodate for these longand short term physiological changes. In an existing device, theAffinity® DR, Model 5330 L/R Dual-Chamber Pulse Generator, manufacturedby the assignee of the present invention, an AutoCapture™ pacing systemis provided. The User's Manual, ©1998 St. Jude Medical, which describesthis capability is incorporated herein by reference. In this system, thethreshold amplitude level is automatically determined for apredetermined duration level in a threshold search routine and captureis maintained by a capture verification routine. Once the thresholdsearch routine has determined a pulse amplitude that will reliablystimulate, i.e., capture, the patient's heart, the capture verificationroutine monitors signals from the patient's heart to identify pulsesthat do not stimulate the patient's heart (indicating aloss-of-capture). Should a loss-of-capture (LOC) occur, the captureverification routine will generate a large amplitude (e.g., 4.5 volt)backup pulse shortly after (typically within 80-100 milliseconds) theoriginal (primary) stimulation pulse. This capture verification occurson a pulse-by-pulse basis and thus, the patient's heart will not miss abeat. However, while capture verification ensures the patient's safety,the delivery of two stimulation pulses (with the second stimulationpulse typically being much larger in amplitude) is potentially wastefulof a limited resource, the battery capacity. To avoid this condition,the existing device, monitors for two consecutive loss-of-capture eventsand only increases the amplitude of the primary stimulation pulse shouldtwo consecutive loss-of-capture (LOC) events occur, i.e., according to aloss-of-capture criteria. This procedure is repeated, if necessary,until two consecutive pulses are captured, at which time a thresholdsearch routine will occur. The threshold search routine decreases theprimary pulse amplitude until capture is lost on two consecutive pulsesand then, in a similar manner to that previously described, increasesthe pulse amplitude until two consecutive captures are detected. This isdefined as the capture threshold. The primary pulse amplitude is thenincreased by a safety margin value, e.g., 0.3 volts, to ensure a primarypulse whose amplitude will exceed the threshold value and thus reliablycapture the patient's heart without the need for frequent backup pulses.In a copending, commonly-assigned U.S. patent application Ser. No.09,483,908 to Paul A. Levine, entitled “An Implantable CardiacStimulation Device Having Autocapture/Autothreshold Capability”,improved loss-of-capture criteria are disclosed which are based upon Xout of the last Y beats, where Y is greater than 2 and X is less than Y.The Levine application is incorporated herein by reference in itsentirety.

Whether a stimulation pulse successfully captures muscle, e.g., cardiac,tissue and thus causes the muscle to contract is related to an amplitudecomponent, i.e., voltage or current, and a duration component of thestimulation pulse. This relationship was described in 1909 by Lapicqueas a strength-duration curve (see an exemplary curve 10 in FIG. 1) whichis expressed by the equation:

I=I _(R)*(1+d _(c) /d)

where I_(R) represents the current at the rheobase, i.e., the lowestcurrent pulse (independent of duration) that can stimulate the bodytissue and d_(c) represents the chronaxie time duration, i.e., aduration at which stimulation requires twice the rheobase current value.

This relationship is readily apparent by setting d equal to d_(c) whichresults in I=2*I_(R).

This equation can be adjusted to display voltage by multiplying eachside by the lead impedance, resulting in:

V=V _(R)*(1+d _(c) /d)

The energy used for each pulse is a function of the amplitude level(i.e., voltage or current) and the duration of the delivered pulse asshown in the equation:

E=(V ² *d)/R

where V is the amplitude of the voltage pulse, d is its duration and Ris the lead impedance.

It has been observed and can be shown that the minimum energy point onthe strength-duration curve is at a chronaxie point 12 (as shown in FIG.1 which shows a prior art implementation of a stimulation energy curve),i.e., where the amplitude component is twice the rheobase 10 and theduration component is the chronaxie duration. Known automaticcapture/threshold algorithms adjust the threshold amplitude, e.g.,voltage, at a fixed duration, preferably the chronaxie duration. Itappears that these algorithms are based on the assumption that changesin the strength-duration curve solely effect the rheobase, i.e., if thechronaxie is essentially fixed, the strength-duration curve will solelyshift vertically during the life of the patient (see curve 14 relativeto curve 10). Since the known existing automatic capture/thresholdalgorithms only alter the amplitude component (see stimulation energycurve 16), the belief that the chronaxie is “fixed” for a given patientis inherent in these algorithms. If in fact the chronaxie is fixed, anamplitude shift alone will result in the minimum energy dissipationsince the stimulation point would shift from the chronaxie point 12 ofstrength-duration curve 10 to the chronaxie point 18 of the subsequentstrength-duration curve 14.

Additionally, it is shown in a copending, commonly assigned PCT Patentapplication No. SE99/00813 to Nils Holmstrom entitled “Variable SafetyMargin in Autocapture Pacemakers,” that due to the shape of thestrength-duration curve, a larger safety margin is desirable withshorter duration stimulation pulses. Accordingly, the strength-durationcurve (see FIG. 2) is divided into two regions having differently sizedsafety margins. The Holstrom application is incorporated herein byreference.

However, in contrast to the belief that the chronaxie was fixed, it hasbeen noted by Raschack in an article entitled: “Differences in thecardiac actions of the calcium antagonists verapamil and nifedipine”Arzneimittelforschung 1976;26 (7):1330-3, that the strength-durationcurve “is shifted to the right and the chronaxia (sic) value issignificantly increased by verapamil.”

The present inventors opines that such a horizontal shift, i.e., achronaxie shift, or a combined horizontal and vertical shift, i.e., ashift in the rheobase and chronaxie, are not optimally accommodated bythe prior art. Additionally, it is noted that since the energydissipation is related to the square of the amplitude (voltage) of astimulation pulse and only linearly related to its duration,amplitude-only increases to regain/maintain capture may be wasteful ofbattery capacity.

The altering of amplitude or duration have been examined in U.S. Pat.No. 5,697,956 to Bornzin, which is incorporated herein by reference. TheBornzin patent recognized that while the selection of stimulation energylevels was ideally related to the strength-duration curve for thepatient's cardiac tissue, optimal increases in energy levels should alsotake into account the battery voltage when voltage doublers (ortriplers) are necessary to achieve a desired stimulation voltage.Accordingly, the Bornzin patent shows a stimulation energy curve (seeFIG. 7 of Bornzin) that selectively increased either amplitude orduration (but not both) to increase the stimulation energy level whileavoiding use of the voltage doublers (or triplers) when possible.However, the Bornzin patent does not show a system in which amplitudeand duration were concurrently increased to increase stimulation energy.

U.S. Pat. No. 4,590,941 to Saulson et al. did show the use ofstimulation pulses where the amplitude and the pulse width components ofstimulation pulses were linearly related. However, Saulson did not showthe use of these pulses in a system which included a method forautomatic capture/threshold determination. In fact, thisamplitude/duration relationship was not used in Saulson to improvecapture of the patient's heart. Specifically, Saulson disclosed a systemin which its programmability was unidirectional and the only way toconfirm the system's programming was to monitor the stimulation pulse'sduration and thus infer the stimulation pulse's amplitude due to thispredefined relationship.

Therefore what is needed is a system that can adjust the amplitude andduration of stimulation pulses to improve immunity to shifts in thestrength-duration curve and thus maintain capture in an automaticcapture/threshold environment while minimizing battery depletion.

SUMMARY OF THE INVENTION

The present invention provides an improved system and method forperforming automatic capture and threshold detection in an implantablecardiac stimulation device. The present invention defines an essentiallylinear stimulation energy curve that predominantly exhibits aconcomitant increase in the amplitude and duration components of thestimulation pulses to achieve and maintain capture and that has anincreased immunity (i.e., a decreased susceptibility) to changes in thechronaxie and/or rheobase. Consequently, the ability to regain capturein the event of a loss-of-capture is improved because it will regaincapture more quickly as well as generally reducing the required energyconsumption. Additional embodiments of the present invention furtheralter the stimulation energy curve to accommodate the use of voltagemultipliers (i.e., doublers or triplers) when necessary.

A preferred implantable cardiac stimulation device is configured forstimulating a patient's heart through at least one electrode implantedin electrical contact with selected cardiac tissue using a pulsegenerator configured for electrical coupling to the electrode andconfigured to generate stimulation pulses at a controlled energy levelto thereby stimulate the patient's heart, wherein the controlled energylevel is defined by a set of characteristics including an amplitudecomponent and a duration component. Additionally, a detection circuit isconfigured for electrical coupling to the electrode and configured toreceive cardiac signals for determining the presence or absence of anevoked response to each of the stimulation pulses. A preferred deviceoperates under control of a controller, coupled to the pulse generatorwhich increases the controlled energy level in response to aloss-of-capture criteria related to the absence of an evoked response.In such a case, the controlled energy level is increased from a firstenergy level to a second energy level where the amplitude component ofthe second energy level exceeds the amplitude component of the firstenergy level and the duration component of the second energy levelexceeds the duration component of the first energy level.

In a significant aspect of the present invention, the relationshipbetween the amplitude and duration components is defined by anessentially linear relationship specified by the equation:

amplitude=(slope*duration)+offset

where the slope is a value in amplitude units/duration units. Forexample, if amplitude is in volts and duration is in milliseconds, theslope is in units of volts/millisecond.

In a further aspect of the present invention, the chronaxie and rheobaseof the strength-duration curve are periodically determined and therelationship between the amplitude and duration components aredetermined accordingly.

In a next aspect of a preferred embodiment of the present invention, thestimulation energy curve is adjusted to additionally include a portionwhere the amplitude component is essentially fixed and the durationcomponent increases to minimize power consumption resulting from the useof a voltage multiplier to generate the required amplitude component.

The novel features of the invention are set forth with particularity inthe appended claims. The invention will be best understood from thefollowing description when read in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a pair of exemplary strength-duration curvehaving fixed chronaxie values and varying rheobase values thatdemonstrate the vertical stimulation energy adjustment curve of theprior art.

FIG. 2 shows a strength-duration curve having a safety margin thatvaries between two levels according to two pulse duration regions.

FIG. 3 shows a simplified functional block diagram of an implantablecardioverter/defibrillator (ICD), which represents one type ofimplantable cardiac stimulation device with which the present inventionmay be used.

FIG. 4 shows a functional block diagram of an implantable dual-chamberpacemaker, which represents another type of implantable medical devicewith which the invention may be used.

FIG. 5 shows an exemplary strength-duration curve and the regions wherecapture does and does not occur.

FIG. 6 shows a strength-duration curve where a plurality of essentiallylinear stimulation energy curves are defined for two pulse durationregions where the stimulation energy curves are angular in a firstregion and vertical in a second region.

FIG. 7 shows a strength-duration curve where a plurality of essentiallylinear stimulation energy curves are defined for three regions where thestimulation energy curves are horizontal in a first region, angular in asecond region and vertical in a third region.

FIG. 8 shows a graphical description of an alternative stimulationenergy curve of the present invention.

FIG. 9 shows a series of strength-duration curves having varyingchronaxie values and rheobase values and a stimulation energy curve thatis optimized for physiological variation and noise immunity, i.e., aminimized susceptibility to variations in chronaxie and/or rheobase aswell as measurement variations.

FIGS. 10 and 11 show an exemplary structure of an amplitude-durationtable that may be used to specify the amplitude and duration componentsof stimulation pulses along a defined stimulation energy curve.

FIG. 12 shows a simplified block diagram of the pertinent features of anexemplary automatic capture/threshold method for use with the preferredstimulation energy curve of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description is of the best mode presently contemplated forcarrying out the invention. This description is not to be taken in alimiting sense, but is made merely for the purpose of describing thegeneral principles of the invention. The scope of the invention shouldbe determined with reference to the claims.

The present invention provides an improved system and method forperforming automatic capture and threshold detection in an implantablecardiac stimulation device, e.g., a pacemaker or an implantablecardioverter/defibrillator (ICD).

To better understand the invention, it will first be helpful to have anunderstanding of the basic functions performed by exemplary implantablestimulation devices with which the invention may be used, e.g., an ICDwith dual chamber coils (see FIG. 3) and/or a dual-chamber pacemaker(see FIG. 4). While a dual-chamber device has been chosen for thisdescription, this is for teaching purposes only. It is recognized thatthe present invention could be implemented into a device having one tofour chambers, that one of skill in the art could readily adapt thedual-chamber device shown in FIG. 4 to perform single ormultiple-chamber functionality, and that a single or multiple chamberdevice is within the spirit of the invention as is any device capable ofdelivering stimulating impulses to a tissue or organ of the body.

In FIG. 3, there is shown a simplified functional block diagram of anICD device 20, and in FIG. 4, there is shown a simplified functionalblock diagram of a dual-chamber pacemaker 70. It should also be notedthat, in some instances, the functions of an ICD and a pacemaker may becombined within the same cardiac stimulation device. However, forteaching purposes, the devices will be described separately.

It is the primary function of an ICD device to sense the occurrence of atachyarrhythmia and to automatically apply an appropriate electricalshock therapy to the heart aimed at terminating the tachyarrhythmia. Tothis end, the ICD device 20, as shown in the functional block diagram ofFIG. 3, includes a control and timing circuit (hereinafter referred toas a control/timing circuit) 22, such as a microprocessor, state-machineor other such control circuitry, that controls a high output chargegenerator 26. The high output charge generator 26 generates electricalstimulation pulses of moderate or high energy (corresponding tocardioversion or defibrillation pulses, respectively), e.g., electricalpulses having energies of from 1 to 10 joules (moderate) or 11 to 40joules (high), as controlled by the control/timing circuit 22.

Such moderate or high energy pulses are applied to the patient's heart28 through at least one lead 30 having at least two defibrillationelectrodes, such as coil electrodes 38 in the atrium and 40 in thesuperior vena cava. The lead 30 preferably also includes at least oneelectrode for pacing and sensing functions, such as electrode 32.Typically, the lead 30 is transvenously inserted into the heart 28 so asto place the coil electrodes 38 and 40 where they are in electrical andpreferably physical contact with the patient's heart 28. While only onelead is shown in FIG. 3, it is to be understood that additionaldefibrillation leads and electrodes may be used to apply the shocktreatment generated by the high voltage generator 26 to the patient'sheart 28.

The ICD 20 also includes a sense amplifier 42 that is coupled to atleast one sensing electrode 32. It is the function of the senseamplifier 42 to sense the electrical activity of the heart 28, as isknown in the art, such as R-waves which are the surface ECGrepresentation of ventricular depolarizations which result in thecontraction of ventricular tissue, and P-waves which are the surface ECGmanifestations of atrial depolarizations which result in the contractionof atrial tissue. Thus, by sensing the ventricular and/or atrialdepolarizations (manifested by the R-waves and/or P-waves on the surfaceECG) through the sense amplifier 42, the control/timing circuit 22 isable to make a determination as to the rate and regularity of thepatient's heart beat. Such information, in turn, allows thecontrol/timing circuit 22 to determine whether the patient's heart 28 isexperiencing an arrhythmia, and to apply appropriate stimulationtherapy. Alternatively, a pacing pulse generator 43 can be used to pacethe heart in accordance with a preselected pacing strategy. Toaccomplish this task, the amplitude of pacing pulses generated by thepulse generator 43 is set by the physician to a value above thethreshold level, e.g., by a fixed value, for the patient's heart toensure capture, i.e., successful stimulation of the patient's heart.

The control/timing circuit 22 further has a memory circuit 44 coupledthereto wherein the operating parameters used by the control/timingcircuit 22 are stored. Such operating parameters define, for example,the amplitude of each shocking pulse to be delivered to the patient'sheart 28 as well as the duration of these shock pulses. The memory 44may take many forms, and may be subdivided into as many different memoryblocks or sections (addresses) as needed to store desired data andcontrol information. A feature of an exemplary ICD 20 is the ability tosense and store a relatively large amount of data as a data record,which data record may then be used to guide the operation of the device,i.e., the present operating mode of the device may be dependent, atleast in part, on past performance data.

Advantageously, the operating parameters of the implantable device 20may be non-invasively programmed into the memory 44 through a telemetrycircuit 46, in telecommunicative contact with an external programmer 48by way of a suitable coupling coil 50. The coupling coil 50 may serve asan antenna for establishing a radio frequency (RF) communication link 52with the external programmer 48, or the coil 50 may serve as a means forinductively coupling data between the telemetry circuit 46 and theexternal programmer 48, as is known in the art. See, e.g., U.S. Pat.Nos. 4,809,697 (Causey, III et al.) and U.S. Pat. No. 4,944,299(Silvian), incorporated herein by reference. Further, such telemetrycircuit 46 advantageously allows status information relating to theoperation of the ICD 20, as contained in the control/timing circuit 22or memory 44, to be sent to the external programmer 48 through theestablished link 52.

The control/timing circuit 22 includes appropriate processing and logiccircuits for analyzing the output of the sense amplifier 42 anddetermining if such signals indicate the presence of an arrhythmia.Typically, the control/timing circuit 22 is based on a microprocessor,controller, or similar processing circuit, which includes the ability toprocess or monitor input signals (data) in a prescribed manner, e.g., ascontrolled by program code stored in a designated area or block of thememory 44. The details of the design and operation of the control/timingcircuit 22 are not critical to the present invention. Rather, anysuitable control/timing circuit 22 may be used that performs thefunctions described herein. The use, design, and operation ofmicroprocessor-based control circuits to perform timing and dataanalysis functions is known in the art.

The ICD 20 additionally includes a battery 62 which provides operatingpower to all of the circuits of the ICD 20. The battery 62 additionallyprovides power to a voltage multiplier 68, e.g., a voltage doubler ortripler, which operates under control of the control timing circuit 22when necessary for providing stimulation voltages in excess of thebattery voltage.

In FIG. 4, a simplified block diagram of the circuitry needed for adual-chamber pacemaker 70 is illustrated. The pacemaker 70 is coupled toheart 28 by way of leads 74 and 76, the lead 74 having an electrode 75that is in electrical and preferably physical contact with one of theatria of the heart 28, and the lead 76 having an electrode 77 that is inelectrical and preferably physical contact with one of the ventricles ofthe heart 28. The leads 74 and 76 are electrically and physicallyconnected to the pacemaker 70 through a connector 73 that forms anintegral part of the housing wherein the circuits of the pacemaker arehoused. Typically, leads 74 and 76 are operated in a bipolar mode wherea “tip” portion provides the voltage signal that provides current thatflows to a “ring” portion on the same lead. Alternatively, leads 74 and76 can operate in a unipolar mode where current flows from the “tip”portion of each lead to a conductive case 116 which surrounds thepacemaker device 70.

The connector 73 is electrically connected to a protection network 79,which network 79 electrically protects the circuits within the pacemaker70 from excessive shocks or voltages that could appear on the electrodes75 and/or 77 in the event such electrodes were to come in contact with ahigh voltage signal, e.g., from a defibrillation shock.

The leads 74 and 76 carry stimulation pulses to the electrodes 75 and 77from an atrial pulse generator (A-PG) 78 and a ventricular pulsegenerator (V-PG) 80, respectively. Further, electrical signals from theatria are carried from the electrode 75, through the lead 74, to theinput terminal of an atrial channel sense amplifier (A-AMP) 82; andelectrical signals from the ventricles are carried from the electrode77, through the lead 76, to the input terminal of a ventricular channelsense amplifier (V-AMP) 84. Similarly, electrical signals from both theatria and ventricles are applied to the inputs of an intracardiacelectrogram amplifier (IEGM) 85. The amplifier 85 is typicallyconfigured to detect an evoked response from the heart 28, i.e., aresponse to an applied stimulation pulse, thereby aiding in thedetection of “capture”. Capture occurs when an electrical stimulusapplied to the heart is of sufficient energy to depolarize the cardiactissue, thereby causing the heart muscle to contract, or in other words,causing the heart to beat. Capture does not occur when an electricalstimulus applied to the heart is of insufficient energy to depolarizethe cardiac tissue. Following each captured stimulation pulse, theassociated cardiac tissue (i.e., the atria or the ventricles) entersinto a physiologic refractory period during which it cannot bere-stimulated.

The dual-chamber pacemaker 70 is controlled by a control and timingcircuit (hereinafter referred to as a control/timing circuit) 86 thattypically includes a microprocessor programmed to carry out control andtiming functions. The control/timing circuit 86 receives the sensedsignals from the atrial (A-AMP) amplifier 82 over signal line 88.Similarly, the control/timing circuit 86 receives the output signalsfrom the ventricular (V-AMP) amplifier 84 over signal line 90, and theoutput signals from the IEGM amplifier 85 over signal line 91. Theseoutput signals which indicate capture due to an evoked response aregenerated each time that a P-wave or an R-wave is sensed within theheart 28. The control/timing circuit 86 also generates trigger signalsthat are sent to the atrial pulse generator (A-PG) 78 and theventricular pulse generator (V-PG) 80 over signal lines 92 and 94,respectively, to control the amplitude and duration of the signalsdelivered to the electrodes, 75 and 77. These trigger signals aregenerated each time that a stimulation pulse is to be generated by therespective pulse generator 78 or 80. The atrial trigger signal isreferred to simply as the “A-trigger”, and the ventricular triggersignal is referred to as the “V-trigger”.

During the time that either an A-pulse or V-pulse is being delivered tothe heart, the corresponding sense amplifier, A-AMP 82 and/or V-AMP 84,is typically disabled by way of a blanking signal presented to theseamplifiers from the control/timing circuit 86 over signal lines 96 and98, respectively. This blanking action prevents the amplifiers 82 and 84from becoming saturated from the relatively large stimulation pulsesthat are present at their input terminals during this time. Thisblanking action also helps prevent residual electrical signals presentin the muscle tissue as a result of the pacemaker stimulation from beinginterpreted as P-waves or R-waves.

As shown in FIG. 4, the pacemaker 70 further includes a memory circuit100 that is coupled to the control/timing circuit 86 over a suitabledata/address bus 102. This memory circuit 100 allows certain controlparameters, used by the control/timing circuit 86 in adjusting orprogramming the operation of the pacemaker 70, to be stored andmodified, as required, in order to customize the pacemaker's operationto suit the needs of a particular patient. Further, data regarding theoperation of the pacemaker 70 (sensed or paced events and activation ofany special algorithms or results of interventions) may be stored in thememory 100 for later retrieval and analysis.

As with the memory 44 of the ICD device 20 shown in FIG. 3, the memory100 of the pacemaker 70 (FIG. 4) may take many forms and may besubdivided into as many different memory blocks or sections (addresses)as needed in order to allow desired data and control information to bestored. A feature of an exemplary cardiac stimulation device is theability to store a relatively large amount of sensed data as a datarecord, which data record may then be used to guide the operation of thedevice. That is, the operating mode of the pacemaker may be dependent,at least in part, on past performance data. For example, an averageatrial rate may be determined based on the sensed atrial rate over aprescribed period of time. This average rate may then be stored andupdated at regular intervals. Such stored rate may then be compared to apresent atrial rate and, depending upon the difference, used to controlthe operating mode of the pacemaker. Other parameters, of course, inaddition to (or in lieu of) atrial rate, may be similarly sensed,stored, averaged (or otherwise processed), and then used for comparisonpurposes against one or more currently-sensed parameters.Advantageously, modern memory devices allow for the storage of largeamounts of data in this manner.

A clock circuit 103 directs an appropriate clock signal(s) to thecontrol/timing circuit 86, as well as to any other needed circuitsthroughout the pacemaker 70 (e.g., to the memory 100) by way of clockbus 105.

A telemetry/communications circuit 104 is further included in thepacemaker 70. This telemetry circuit 104 is connected to thecontrol/timing circuit 86 by way of a suitable command/data bus 106. Inturn, the telemetry circuit 104, which is included within theimplantable pacemaker 70, may be selectively coupled to the externalprogrammer 48 by means of the communication link 52. The communicationlink 52 may be any suitable electromagnetic link such as an RF (radiofrequency) channel, a magnetic link, an inductive link, an optical link,and the like. Advantageously, desired commands may be sent to thecontrol/timing circuit 86 through the external programmer 48 and thecommunication link 52. Similarly, through this communication link 52with the programmer 48, data commands (either held within thecontrol/timing circuit 86, as in a data latch, or stored within thememory 100) may be remotely received from the programmer 48. Similarly,data initially sensed through the leads 74 or 76, and processed by thecontrol/timing circuit 86, or other data measured within or by thepacemaker 70, may be stored and uploaded to the programmer 48. In thismanner, non-invasive communications can be established with theimplanted pacemaker 70 from a remote, non-implanted, location.

The pacemaker 70 additionally includes a battery 93 which providesoperating power to all of the circuits of the pacemaker 70 via a powersignal line 95. The battery 93 additionally provides power to a voltagemultiplier 118, e.g., a voltage doubler or tripler, which operates undercontrol of the control/ timing circuit 86 when necessary for providingstimulation voltages in excess of the battery voltage.

It is noted that the pacemaker 70 in FIG. 4 is referred to as adual-chamber pacemaker because it interfaces with both the atria and theventricles of the heart 28. Those portions of the pacemaker 70 thatinterface with the atria, e.g., the lead 74, the P-wave sense amplifier82, the atrial pulse generator 78, and corresponding portions of thecontrol/timing circuit 86, are commonly referred to as the “atrialchannel”. Similarly, those portions of the pacemaker 70 that interfacewith the ventricles, e.g., the lead 76, the R-wave sense amplifier 84,the ventricular pulse generator 80, and corresponding portions of thecontrol/timing circuit 86, are commonly referred to as the “ventricularchannel”. While a dual chamber pacemaker includes a single atrialchannel and a single ventricular channel, multichamber devices mayinclude more than one atrial channel and/or more than one ventricularchannel.

As needed for certain applications, the pacemaker 70 may further includeat least one sensor 112 that is connected to the control/timing circuit86 of the pacemaker 70 over a suitable connection line 114. While thissensor 112 is illustrated in FIG. 4 as being included within thepacemaker 70, it is to be understood that the sensor may also beexternal to the pacemaker 70, yet still be implanted within or carriedby the patient. A common type of sensor is an activity sensor, such as apiezoelectric crystal, that is mounted to the case of the pacemaker.Other types of sensors are also known, such as sensors that sense theoxygen content of blood, respiration rate, pH of blood, body motion, andthe like. The type of sensor used is not critical to the presentinvention. Any sensor or combination of sensors capable of sensing aphysiological or physical parameter relatable to the rate at which theheart should be beating (i.e., relatable to the metabolic need of thepatient), and/or relatable to whether a tachyarrhythmia is likely tosoon occur, can be used. Such sensors are commonly used with“rate-responsive” or “rate-modulated” pacemakers in order to adjust therate (pacing cycle) of the pacemaker in a manner that tracks thephysiological or metabolic needs of the patient.

The pacemaker 70 further includes magnet detection circuitry 87, coupledto the control/timing circuit 86 over signal line 89. It is the purposeof the magnet detection circuitry 87 to detect when a magnet is placedover the pacemaker 70. The magnet may be used by a physician or othermedical personnel to perform various reset functions of the pacemaker 70and/or to signal the control/timing circuit 86 that an externalprogrammer 48 is in place to receive data from, or send data to, thepacemaker memory 100 or control/timing circuit 86 through the telemetrycommunications circuit 104.

As with the ICD device 20 of FIG. 3, the telemetry or communicationscircuit 104 may be of conventional design, such as is described in U.S.Pat. No. 4,944,299, or as is otherwise known in the art. Similarly, theexternal programmer 48 may be of any suitable design known in the art,such as is described in U.S. Pat. No. 4,809,697. Likewise, the memorycircuit 100, and the circuits utilized in the atrial and ventricularchannels may all be of common design as is known in the pacing art. Thepresent invention is not concerned with the details of the circuitryutilized for each of these pacing elements. Rather, it is concerned withthe manner in which the amplitude and the duration (width) of the pacingpulses delivered to the heart are determined in coordination withautomatic capture and threshold modes of operation. Such determinationis controlled by the control/timing circuit 86.

The control/timing circuit 86 may be realized using a variety ofdifferent techniques and/or circuits as known in the art. The preferredtype of control/timing circuit 86 is a microprocessor-basedcontrol/timing circuit. It is noted, however, that the control/timingcircuit 86 could also be realized using, for example, a state machine.Indeed, any type of control/timing circuit, controller or system couldbe employed for the control/timing circuit 86. The present invention islikewise not concerned with the details of the control/timing circuits22 and 86. Rather, it is concerned with the end result achieved by thecontrol/timing circuit. That is, so long as the control/timing circuit86 controls the operation of the pacemaker (or other medical device) sothat the desired functions are achieved as set forth herein, it matterslittle what type of control/timing circuit is used. Those of skill inthe implantable medical device art, given the teachings presentedherein, should thus be able to fashion numerous different types ofcontrol/timing circuits that achieve the desired device control.

Representative of the types of control/timing circuits that may be usedwith the invention is the microprocessor-based control/timing circuitdescribed in U.S. Pat. No. 4,940,052, entitled “MicroprocessorControlled Rate-Responsive Pacemaker Having Automatic Rate ResponseThreshold Adjustment”. Reference is also made to U.S. Pat. Nos.4,712,555 and 4,944,298, wherein a state-machine type of operation for apacemaker is described; and U.S. Pat. No. 4,788,980, wherein the varioustiming intervals used within the pacemaker and their inter-relationshipare more thoroughly described. The '052, '555, '298 and '980 patents areincorporated herein by reference.

The strength-duration curve described by Lapicque teaches that theability to depolarize, i.e., cause a muscle to contract, is a functionof both the amplitude and duration of a stimulation pulse and not justthe overall energy level. For example, if a stimulation pulse isprovided with an amplitude below the rheobase but at infinite duration(and thus infinite energy), the muscle will still not be stimulated.Similarly, if a stimulation pulse is provided with an extremely largeamplitude but at a very small duration, there will be no stimulation.Graphically, it may be seen that points below (i.e., downward and to theleft) the strength-duration curve (see Region A of FIG. 5) will notresult in stimulation and that points above (i.e., upward and to theright) or on the strength-duration curve will result in stimulation (seeRegion B of FIG. 5). Furthermore, it is believed that increasing thegraphical distance (i.e., the graphically viewed variation in amplitudeand duration as will be shown in reference to FIG. 8) of a stimulationenergy point away from the strength-duration curve in Region B increasesthe immunity (i.e., decreases the susceptibility) of the system tomeasurement and stimulation pulse variations and variations in thechronaxie and rheobase which will reposition the strength-durationcurve.

Accordingly, in a first embodiment of the present invention as shown inFIG. 6, it is observed that the strength-duration curve 600 isrelatively flat in Region II, i.e., the pulse duration region above afirst duration threshold 602, e.g., the chronaxie duration. Accordingly,the maximum immunity to variations is achieved by solely increasing theamplitude of stimulation pulses in Region II since the relative effectof duration increases are minimal. However, in Region I, i.e., the pulseduration region below the first duration threshold 602, the immunity ispreferably increased by increasing both the amplitude and durationcomponents or, in an alternative embodiment, increasing the durationcomponent by one step (as a minimum) while the amplitude increases canrange from 0-2 steps, for example. The first duration threshold 602 maybe predefined, programmable via the external programmer 48, or, asdiscussed further below, the chronaxie (and rheobase) may beperiodically calculated and this calculated chronaxie value (or a valuerelated to this value) may be used as the first duration threshold 602.

In a second embodiment of the present invention as shown in FIG. 7, itis further observed that there is little consequence of increasing theamplitude in Region I due to the predominantly vertical slope of thestrength-duration curve within that region. Accordingly, increases instimulation energy in Region I predominantly occur by increasing thepulse duration. Region I may be defined by an amplitude threshold 604.For example, the amplitude threshold 604 may be a value related to therheobase value, e.g., 3*rheobase, or may be defined by a second durationthreshold 606, e.g., 0.5*chronaxie. In Region III, in a manner similarto that previously described for Region II in FIG. 6, the stimulationenergy is increased by predominantly increasing the amplitude component.Region III is defined by a third duration threshold 608, e.g., a value50% greater than the chronaxie duration. In Region II, both theamplitude and duration components are increased, preferably linearly, aspreviously described in relationship to Region I in FIG. 6. Aspreviously described, these thresholds may be predefined, programmablefrom the external programmer 48 or may be automatically set as a resultof a periodic chronaxie/rheobase calculation.

A third embodiment, graphically illustrated in FIG. 8, incorporates allof the prior reasoning in a single, optimal solution. It is believedthat a stimulation energy curve that is equidistant from all points onthe strength-duration curve will present the maximum immunity (i.e.,minimized susceptibility) to measurement and stimulation pulsevariations and variations in the chronaxie and rheobase which willreposition the strength-duration curve. In a first variation, thisstimulation energy curve (see stimulation energy curve 700) intersectsthe strength-duration curve at its chronaxie point 702 so that thelowest energy capture point of the strength-duration curve and thelowest energy capture point of the stimulation energy curve coincide. Atthis intersection point, the distance of the stimulation energy curvefrom the strength-duration curve will be, by definition, zero inamplitude and in duration.

Stimulation energy curve 700 (see FIG. 8 which graphs amplitude in voltsvs. duration in milliseconds) has a differential increase in amplitude(e.g., voltage V_(T) in volts) and duration d_(T) (in milliseconds)equal to each other at all points on the stimulation energy curve. Thus,for a unity slope: $\begin{matrix}{{\Delta \quad V} = \quad {\Delta \quad d}} \\{{\Delta \quad V} = {{V_{T} - V_{C}} = \quad {d_{T} - d_{C}}}}\end{matrix}$

where V_(C) is the voltage at the chronaxie duration (d_(c)) which is,by definition, twice the rheobase (V_(R)). Thus,

ΔV=V _(T)−(2*V _(R))=d _(T) −d _(C)

Thus,

V _(T) =d _(T)+((2*V _(R))−d _(C))  Equation 1

i.e., the preferred stimulation energy curve is represented by anequation of the form:

amplitude=(slope*duration)+offset

where the slope is preferably 1.0 volts/millisecond and the offset value(in volts) is preferably:

(2*rheobase)−chronaxie

where the rheobase value is in volts and the chronaxie value is inmilliseconds.

This stimulation energy curve presumes an ideal stimulation pulsegenerator (i.e., a pulse generator that is not subject to quantizationeffects) and a strength-duration curve that is equally susceptible tochronaxie and rheobase variations. To accommodate departures from thesepresumptions, the slope of a preferred stimulation energy curve may bewithin the range of 0.3 to 3.0 volts/millisecond. Furthermore, toaccommodate actual variations in the strength-duration curve, that slopemay be adjusted according to measured trends in the strength-durationcurve, i.e., measured variations in the chronaxie and/or rheobasebetween two or more measurements.

However, it has been determined that the chronaxie point 702 is not thegraphical center of the strength-duration curve and that the graphicalcenter of the strength-duration curve is slightly to the right of theactual chronaxie point 702. This point will be referred to as aquasi-chronaxie point 703 since it bears some resemblance to thechronaxie point but is materially different in that it has been selectedfor its graphical significance, despite not being the minimum energypoint on the strength-duration curve. In a second variation, astimulation energy curve 704 passes through this quasi-chronaxie point703. Stimulation energy curve 704 may be represented by the equation(where amplitude and rheobase are in units of volts and duration is inunits of milliseconds, i.e., its slope of 1.0 is in units ofvolts/millisecond):

amplitude=duration+rheobase  Equation 2

While this stimulation energy curve 704 will not provide the minimumstimulation energy at its intersection with the strength-duration curve(since it will not intersect the actual chronaxie point 702), it willbisect the strength-duration curve and thus more accurately represent astimulation energy curve that is equidistant from the strength-durationcurve.

FIG. 9 shows the use of this stimulation energy curve on a range ofvarying strength-duration curves having the following attributes(assuming a 500 ohm lead impedance):

Chronaxie Rheobase CURVE (milliseconds) (volts) A 0.30 0.35 B 0.60 0.70C 0.60 0.35 D 0.30 0.70

Strength-duration curve A is presumed to be the initialstrength-duration curve for this analysis and curves B-D are exemplaryvariations that may occur during the operation of the cardiacstimulation device due to physiological variations. Accordingly, thestimulation energy curve 800 corresponding to the strength-durationcurve A (with V_(T) in volts and d_(T) in milliseconds) is defined by(see Equation 1):

V _(T) =d _(T)+0.40

Initially, it may be noted graphically that the shortest graphicaldistance between the chronaxie 802 of curve A, the initial chronaxiepoint, and the stimulation threshold of any of the other curves is alongthe stimulation energy curve 800. Thus, a first consequence of thisstimulation energy curve is that should capture be lost, it will be ableto be regained in a minimal period of time. This will limit the need forbackup pulses which dissipate a “large” amount of power and may causediscomfort to some patients.

The following tables illustrate the energy effects of regaining capturealong the preferred stimulation energy curve as opposed to the prior artwhich used a unidirectional, i.e., amplitude or pulse width only,stimulation energy curve. While energy is shown in the following tables,this is for illustration purposes only since the present invention doesnot require energy calculations to perform its function.

TABLE 1 Transition from curve A to curve B (double the rheobase anddouble the chronaxie) Amplitude Duration Energy (volts) (milliseconds)(micro joules) Chronaxie A 0.7 0.3 0.294 Chronaxie B 1.4 0.6 2.352Preferred Curve 1.215 0.815 2.406 Prior Art 2.1 0.3 2.646

In this example, the present invention will save approximately 9% inenergy per pulse and will recapture faster.

TABLE 2 Transition from curve A to curve C (double the chronaxie onlywith a constant rheobase) Amplitude Duration Energy (volts)(milliseconds) (micro joules) Chronaxie A 0.7 0.3 0.294 Chronaxie C 0.70.6 0.588 Preferred Curve 0.834 0.434 0.604 Prior Art 1.05 0.3 0.662

In this example, the present invention will save approximately 9% inenergy per pulse and will recapture faster.

TABLE 3 Transition from curve A to curve D (double the rheobase onlywith a constant chronaxie) Amplitude Duration Energy (volts)(milliseconds) (micro joules) Chronaxie A 0.7 0.3 0.294 Chronaxie D 1.40.3 1.176 Preferred Curve 1.032 0.632 1.346 Prior Art 1.4 0.3 1.176

In this example, the present invention will use approximately 14% moreenergy per pulse. However, it will recapture faster.

Accordingly, as expected, in an environment where the chronaxie isconstant, the present invention will recapture faster at the cost ofsome additional energy dissipation. However, in other environments, thepresent invention will recapture faster and save energy. Mostsignificantly, the ability to retain capture at a given energy levelwill be optimized in the present invention since stimulation pulses willbe selected that increase immunity to measurement variations, bothamplitude and duration variations of the stimulation pulses, andvariations in the strength-duration curve caused by variations in therheobase and/or chronaxie.

In embodiments of the present invention, the stimulation energy isincreased, when necessary along the stimulation energy curve. Increasesin stimulation energy may be made at predetermined distances along thestimulation energy curve 800. Altematively, the stimulation energyincreases may be made in predetermined increments of energy,predetermined increments in amplitude, predetermined increments induration, etc. These calculations may be made “on the fly” by thecontrol/timing circuit 86.

Alternatively, a table 900, as shown in FIGS. 10 and 11, may bepre-populated by pairs of amplitude/duration components values for arange of energy values according to the previously described equations,respectively Equations 1 and 2. For example, FIG. 10 is prepopulatedaccording to the equation:

amplitude=duration+0.40

and FIG. 11 is prepopulated according to the equation:

amplitude=duration+0.35

where amplitude is in volts and duration is in milliseconds.

FIG. 10 shows exemplary data according to the equation for thestimulation energy curve corresponding to the strength-duration curve Afor a stimulation energy curve that intersects the chronaxie point 802while FIG. 11 shows exemplary data for an alternative stimulation energycurve 801 that intersects the quasi-chronaxie point 803. In the case ofFIG. 10, the chronaxie point 802 may be used as a starting controlledenergy level and this energy level may be adjusted upwards by a safetymargin to determine an initial controlled energy level.

Alternatively, in the case of FIG. 11, the quasi-chronaxie point 803 maybe used as the starting controlled stimulation energy level. This datamay be generated in advance at or by the external programmer 48 anddownloaded into the table 900 or may be periodically generated by thecontrol/timing circuit 86. The rheobase and chronaxie values may bemanually entered at the external programmer and these values may be usedin determining the data in table 900. Preferably, as described furtherbelow, the rheobase and chronaxie may be periodically calculated byembodiments of the present invention. Thus, the preferred stimulationenergy curve can be recalculated following this periodic determination,e g., as background operation of the control/timing circuit.

A first alternative technique for determining the chronaxie and rheobasewill now be described.

An equation for approximating then relationship between amplitude andduration for stimulating body, e.g., cardiac, tissue was defined in 1909by Lapicque as a strength-duration curve. The Lapicque equation is:

I=I _(R)*(1+d _(c) /d)

where I_(R) represents the current at the rheobase, i.e., the lowestcurrent signal, independent of duration that can stimulate the bodytissue and d_(c) represents the chronaxie time duration, i.e., aduration at which stimulation occurs at twice the rheobase value.

This relationship is readily apparent by setting d equal to d_(c) whichresults in the equation:

I=2*I _(R)

This equation can be adjusted to display voltage by multiplying eachside by the lead impedance, resulting in:

V=V _(R)*(1+d _(c) /d)

The chronaxie and rheobase may be calculated using the present device.As described below, this calculation may be done using only two sets ofmeasurements.

The amount of charge Q that is needed to stimulate the body tissue canbe expressed as:

 Q=I*d.

where I is current and d is the pulse duration.

Since the stimulation current can be expressed as:

I=I _(R)*(1+d _(c) /d)

We know that: $\begin{matrix}{Q = {I_{R}*\left( {1 + {d_{c}/d}} \right)*d}} \\{= {\left( {I_{R}*d} \right) + \left( {I_{R}*d_{c}} \right)}} \\{= {I_{R}*\left( {d + d_{c}} \right)}}\end{matrix}$

If a fixed pulse duration is picked and a stimulation pulse is emitted,e.g., from ventricular pulse generator 80, the ventricular senseamplifier 84, can look for an evoked response. An evoked response willtypically occur within a window of 15 to 50 milliseconds. If an evokedresponse does not occur, the amplitude of the stimulation pulse isincreased, e.g., by a relatively small (fine) quantity, and the test isrepeated. When an evoked response is detected, a point on thestrength-duration curve has been found.

If this test process is repeated twice, one can arithmetically derivethe rheobase and the chronaxie. For example, the tests may be repeatedat the exemplary values of 1.0 and 2.0 milliseconds (one of ordinaryskill in the art can adapt these calculations for other test values.See, for example, Equations 2 and 3 of U.S. Pat. No. 5,447,525 to Powellet al.) and provide the following Q values:

Q ₂ =I _(R)*(2+d _(c))=(2*I _(R))+(I _(R) *d _(c))

Q ₁ I _(R)*(1+d _(c))=I _(R)+(I _(R) *d _(c))

Accordingly:

Q ₂ −Q ₁ =I _(R)

i.e., the rheobase current is the difference between the two charges(where Q is in millicoulombs and I is in amperes). If the equation ismultiplied by resistance R and the charges are adjusted for their 2.0millisecond and 1.0 millisecond durations, we determine that:

 V _(R)=(2*V ₍₂₎)−V ₍₁₎

i.e., the rheobase voltage can be calculated from the two measuredvoltages at which capture occurred where V₍₂₎ is the measured capturevoltage at 2.0 milliseconds and V₍₁₎ is the measured capture voltage at1.0 milliseconds.

Further substituting the solved rheobase value, (2*V₍₂₎)−V₍₁₎, in theLapicque voltage equation at 1.0 milliseconds, we determine that:

V=V _(R)*(1+d _(c) /d)

V ₁=((2*V ₍₂₎)−V ₍₁₎)*(1+d _(c)/1)

V ₁=((2*V ₍₂₎)−V ₍₁₎)*(1+d _(c))

And thus, with measurements made at 1.0 and 2.0 milliseconds:

d _(c)=(V ₁/((2*V ₍₂₎)−V ₁))−1

In a second alternative technique, the rheobase can be approximated byobserving that typically the Lapicque curve is essentially flat at orbeyond the 2.0 millisecond point. Thus, if a voltage capture level isobtained at or beyond that point, it will approximate the rheobase.Next, using twice the approximated rheobase value, the pulse durationcan be incremented from a starting point, e.g., 0.5 milliseconds, untilan evoked response occurs within the detection window. This point ofcapture may then identified as the chronaxie value. Altematively, sincethe measured capture voltage at 2.0 milliseconds is an approximation ofthe rheobase value, the calculated chronaxie value may be adjusted by apercentage, e.g., 20%, to accommodate the rheobase approximation. In afurther alternative, the measured capture voltage (used to approximatethe rheobase) can be adjusted by a percentage, e.g., 90%, to moreclosely approximate the actual rheobase, and this adjusted value may beused to determine the chronaxie value.

In the event that a voltage multiplier (e.g., a doubler or tripler) isneeded, it has been disclosed in the aforementioned U.S. Pat. No.5,697,956 to Bornzin that the use of the voltage multiplier can bepostponed if portions of the stimulation energy curve increase theduration component instead of the amplitude component. Accordingly, theaforedescribed stimulation curve 700 of FIG. 8 may be modified toaccommodate a voltage multiplier. Accordingly, in the example shown inFIG. 8, the stimulation curve 700 is used, as described above, whichtransitions to a “pulse duration only” adjustment mode above apredetermined voltage threshold, e.g., 2.5 volts. This threshold beingchosen so as to provide a guard band to the voltage multiplier 68 toprevent stimulation pulses that might be voltage limited. One of skillin the art could select any voltage threshold for switching to the pulseduration only mode to optimize current drain, while taking into accountany hardware (e.g., the present battery voltage) or softwarelimitations. Furthermore, the stimulation energy curve may be adjusted(as shown in reference to the aforementioned FIG. 7 of U.S. Pat. No.5,697,956 to Bornzin) to accommodate portions in which amplitudedecreases occur in conjunction with pulse duration increases (see points6 and 14 of Bornzin's FIG. 7 and its associated description).

Accordingly, if a voltage greater than the determined threshold voltageis needed, it may be energy efficient to postpone use of the voltagemultiplier 68 and to modify the stimulation energy curve 700 to includea jog 804 where the duration component increases at a fixed voltage, ator below the battery voltage. After the duration component has beenallowed to increase by fixed amount or percentage, the amplitudecomponent is then allowed to increase with a corresponding decrease inthe duration component by following jog 806. Accordingly, the energylevel at point 808 may be selected to be greater than (or optionallyidentical to) the energy level at point 810. A similar set of jogs, 805and 807, may be included when a voltage tripler is used. Preferably, thecontrol/timing circuit 86 periodically monitors the battery level usinga voltage detector 120 and periodically adjusts the positions of thesejogs along the stimulation energy curve 700 accordingly. While thisadjustment for a voltage multiplier 68 has been shown in reference tostimulation energy curve 700, a similar adjustment is equally applicableto stimulation energy curve 704 which intersects the quasi-chronaxiepoint.

FIG. 12 shows a simplified portion of an exemplary automaticcapture/threshold method for use with the preferred stimulation energycurve. Periodically, e.g., every six to twenty-four hours, thecontrol/timing circuit 86 determines in step 950 whether it is time todetermine the present chronaxie and rheobase values. In step 952, thechronaxie/rheobase determination is made as described above. In step954, the amplitude/duration table 900 is loaded with pairs of pointsaccording to the one of the aforedescribed stimulation energy curves.The pairs of points may be loaded such that they represent fixedincreases in energy, fixed increases in voltage (as shown in theexemplary data in the tables of FIGS. 10 and 11) or duration, orpercentage increases in energy, i.e., increases in energy that arelarger at higher energy levels. Optionally, in step 956, theamplitude/duration table 900 is modified to reflect the present batteryvoltage and the use of a voltage multiplier. Finally in step 958, anenergy point is selected beyond the chronaxie point along thestimulation energy curve (curve 800 in this example) to accommodate asafety margin. This point may be a fixed energy amount greater than thechronaxie point 802, e.g., one or two points further than the chronaxiepoint in table 900, or a percentage amount determined by the energylevel of the current chronaxie point. Additionally, the percentageamount may be programmable from the external programmer 48.

A simplified example of the utility of this concept can be shown inrelationship to a fixed duration stimulation energy curve embodiment.For example, in the prior art, a safety margin of 0.3 volts is typicallyused at the point where capture is achieved. Thus, if capture wasachieved at a voltage of 0.6 volts, a subsequent stimulation pulse woulduse 0.9 volts, i.e., resulting in a safety margin percentage of 50%.However, if capture was achieved at a voltage of 1.0 volts, a subsequentstimulation pulse would use 1.3 volts, i.e., the safety marginpercentage would have decreased to 30%. The decrease in safety marginrelative to stimulation energy which varies according to the square ofthe voltage would be even more significant. Accordingly, embodiments ofthe present invention preferably maintain the safety margin percentagein amplitude or power. For example, if the safety margin is maintainedas an amplitude percentage, the 1.0 volt capture level would increase to1.5 volts.

Following the determination of the chronaxie point 802 plus a safetymargin, this point 812 (from the amplitude duration table 900 orcalculated as needed by the control/timing circuit 86) is used as thepresent stimulation pulse when not inhibited by an intrinsic cardiacevent. The pacemaker 70, as a parallel task, monitors for the presenceof an evoked response to each delivered stimulation pulse. However,should a stimulation pulse at this energy level not capture the cardiactissue in step 960 (as described further in the User's Manual, ©1998 St.Jude Medical for the Affinity® DR, Model 5330 L/R Dual-Chamber PulseGenerator and in a copending, commonly-assigned application to Paul A.Levine, entitled “An Implantable Cardiac Stimulation Device HavingAutocapture/Autothreshold Capability”), a high voltage, e.g., 4.5 volt,backup pulse is generated to ensure capture in step 962. Should aloss-of-capture criteria be met in step 964, e.g., two consecutivelosses-of-capture, the stimulation energy level is increased in step966, preferably by a relatively large (coarse) amount, according to thestimulation energy curve 800. Capture is rapidly regained due to thestimulation energy curve 800 of the present invention, thus minimizingthe number of consecutive cardiac cycles requiring backup pulses. Oncecapture has been regained, the process preferably continues at step 952where the chronaxie and rheobase are redetermined. This process is alsoapplicable to the use of stimulation energy curve 801 which passesthrough the quasi-chronaxie point 803.

Finally, a simplified example is described relative to theamplitude/duration data of table 900 in FIG. 10 and the A and Bstrength-duration curves of FIG. 9. In this example it is assumed thatstrength-duration curve A was initially determined in step 952 and thatthe data of table 900 was generated accordingly in step 954. In thisexample, the data was generated with fixed amplitude increases andcorrespondingly fixed duration increases accordingly to theaforedescribed stimulation energy equation (see Equation 1):

amplitude=duration+0.35

However, fixed duration increases with fixed amplitude increasesaccording to the aforedescribed equation or fixed stimulation energyincreases could have also been used. In this case the original chronaxiepoint 802 was determined to be at an amplitude of 0.70 volts and aduration of 0.30 milliseconds, corresponding to data point 6 (thestarting controlled energy level). To accommodate a safety margin, datapoint 8 (shown as 812 in FIG. 9) is used for the initial stimulationenergy level. For purposes of this illustration, we assume thatphysiological changes have resulted in the cardiac tissue now respondingto strength-duration curve B (corresponding to double the chronaxie anddouble the rheobase from that originally measured for curve A).Accordingly, it is apparent, graphically, that capture will no longeroccur. To regain capture, the stimulation energy level is increasedalong the stimulation energy curve 800 until capture is regained at datapoint 17 (shown as point 814 in FIG. 9). Due to the quantization amountused, i.e., the step size, data point 16 just misses capture and thusdata point 17 must be used. Thus, it may be observed that the efficiencyof this implementation is somewhat quantization dependent.Alternatively, if the control/timing circuit 86 performs thiscalculation as needed without a table, these quantization effects may belimited. Finally, to achieve a safety margin, data point 19 is used asthe new stimulation energy level (shown as point 816 in FIG. 9).

In the prior art, only the amplitude of the stimulation voltage wouldhave been changed to regain capture. Accordingly, capture would havebeen regained at point 818 (2.1 volts, 0.3 milliseconds) and after asafety margin was added, a stimulation energy point 820 (2.4 volts, 0.3milliseconds) would have been used. Various graphical observations cannow be made. First, point 820 is much closer to curve B than point 816.Second, point 816 is much closer to the original chronaxie point 802than is point 820. Accordingly, capture can be regained faster and withbetter immunity to measurement variations and variations in chronaxieand rheobase than possible with the prior art. Despite theseimprovements, the amount of stimulation energy used by points 816 and820 are essentially identical. If it were not for the aforementionedquantization effects relative to this particular example, there alsowould have been an energy saving.

Point 816 is only approximately equidistant in amplitude and durationfrom curve B. This occurs since stimulation energy curve 800 wasdetermined to generate points equidistant in amplitude and duration fromthe original strength-duration curve, i.e., curve A. Accordingly, steps952-958 (see FIG. 12) may be processed to determine a new stimulationenergy curve 809 and a new stimulation energy point 822 that isoptimized for the physiologically altered strength-duration curve B.

Furthermore, a stimulation energy curve that intersects thequasi-chronaxie point 803 may be used, as reflected in FIG. 11. In thisparticular example, the original operating data point (9) (see FIG. 11)will dissipate a greater amount of power than in the case of FIG. 10.However, the final operating data point (19) (see FIG. 11) will resultin a greater energy saving.

Accordingly, what has been shown is an improved stimulation energy curvefor performing an automatic capture/threshold procedure in animplantable stimulation device, e.g., for stimulating cardiac or othermuscle tissue. While the invention has been described by means ofspecific embodiments and applications thereof, it is understood thatnumerous modifications and variations could be made thereto by thoseskilled in the art without departing from the spirit and scope of theinvention. For example, it is believed that the during the acute phasefollowing lead implantation, that shifts in the strength-duration curvepredominantly result from rheobase variations. Accordingly during theacute phase, it may be desirable to automatically (or in response to aninstruction from the external programmer) disable the stimulation energycurve of the present invention (i.e., a stimulation energy curve inwhich most portions include concomitant increases in amplitude andduration) and to operate with a unidirectional, e.g., vertical,stimulation energy curve as found in the prior art (e.g., a stimulationenergy curve in which the duration component is essentially fixed andonly the amplitude component increases). Furthermore, it is recognizedthat a particular implementation of a stimulation device may havequantization restrictions that limit its ability to implement thepreferred linear stimulation energy curve. Thus, an approximation of thepreferred linear stimulation energy curve is considered to be within thescope of the present invention. For example, stepwise approximations ofa stimulation energy curve, e.g., 809, (see step portion 824 whichincreases amplitude and then duration and step portion 826 whichincreases duration and amplitude as shown in FIG. 9) of the preferredlinear stimulation energy curve are also considered to be within thescope of the present invention since such approximations (independent ofthe ratio of the amplitude and duration steps) are considered to beessentially defined by the preferred linear relationship, i.e.,

amplitude=(slope*duration)+offset.

Furthermore, any stimulation energy adjustment for regaining capturealong the aforementioned stimulation energy curve, independent of theinitial starting energy point, is likewise considered to be within thescope of the present invention. It is therefore to be understood thatwithin the scope of the claims, the invention may be practiced otherwisethan as specifically described herein.

What is claimed is:
 1. An implantable stimulation device configured forstimulating muscle tissue through at least one electrode implanted inelectrical contact with selected muscle tissue, the stimulation devicecomprising: a pulse generator configured for electrical coupling to theelectrode and configured to generate stimulation pulses at a controlledenergy level to thereby stimulate the muscle tissue, wherein thecontrolled energy level is defined by a set of characteristics includingan amplitude component and a duration component; a detection circuitconfigured for electrical coupling to the electrode and configured toreceive signals for determining the presence or absence of an evokedresponse to each of the stimulation pulses; a controller, coupled to thepulse generator and the detection circuit, for increasing the controlledenergy level from a first energy level to a second energy level inresponse to a loss-of-capture criteria related to the absence of anevoked response to at least one stimulation pulse generated at the firstenergy level; and wherein the first energy level has a first amplitudecomponent and a first duration component and the second energy level hasa second amplitude component and a second duration component, whereinthe second amplitude component exceeds the first amplitude component andthe second duration component exceeds the first duration component. 2.The implantable stimulation device of claim 1 wherein the set ofcharacteristics for each energy level is determined by the controller.3. The implantable stimulation device of claim 1 wherein the set ofcharacteristics for each energy level is predetermined.
 4. Theimplantable stimulation device of claim 1 wherein the set ofcharacteristics for each energy level is remotely programmable.
 5. Theimplantable stimulation device of claim 1 wherein the relationshipbetween the amplitude and duration components is predominantly definedby an essentially linear relationship specified by the equation:amplitude=(slope*duration)+offset where slope and offset are constantvalues.
 6. The implantable stimulation device of claim 5 wherein theslope is between 0.3 and 3.0 volts/millisecond.
 7. The implantablestimulation device of claim 6 wherein the slope is essentially 1.0volts/millisecond and the offset is a function of a strength-durationcurve characterized by a chronaxie and a rheobase.
 8. The implantablestimulation device of claim 7 wherein values corresponding to thechronaxie and rheobase are remotely programmable.
 9. The implantablestimulation device of claim 7 wherein the controller periodicallydetermines the chronaxie and rheobase.
 10. The implantable stimulationdevice of claim 7 wherein the value of the offset is defined by theequation: offset=(2*rheobase)−chronaxie where rheobase is in volts andchronaxie is in milliseconds.
 11. The implantable stimulation device ofclaim 7 wherein the offset is defined by the equation: offset=rheobase.12. The implantable stimulation device of claim 1 additionallycomprising a table for defining a plurality of energy levels defined byamplitude and duration components.
 13. The implantable stimulationdevice of claim 12 wherein the energy levels defined within the tableinclude at least one pair of sequential energy levels where theamplitude component is essentially the same between the sequentialenergy levels and the duration component increases between thesequential energy levels.
 14. The implantable stimulation device ofclaim 12 wherein the energy levels defined within the table include atleast one pair of sequential energy levels where the amplitude componentincreases between sequential energy levels and the duration componentdecreases between sequential energy levels.
 15. The implantablestimulation device of claim 9 wherein a starting controlled energy levelhaving an amplitude component and a duration component is determined inresponse to the strength-duration curve and an initial controlled energylevel for delivery by the pulse generator is determined by increasingboth amplitude and duration components of the starting controlled energylevel to achieve a safety margin from the determined strength-durationcurve.
 16. The implantable stimulation device of claim 15 wherein theinitial controlled energy level is greater than the starting controlledenergy level by a variable safety margin.
 17. A method for stimulating apatient's body tissue, the method comprising the steps of: periodicallydelivering a stimulation pulse to the patient's body tissue, thestimulation pulse having a controlled energy level, wherein thecontrolled energy level is defined by a set of characteristics includingan amplitude component and a duration component; detecting the presenceor absence of an evoked response generated by the patient's body tissuein response to the stimulation pulse during a detection window;determining whether a loss-of-capture criteria is satisfied in responseto the absence of an evoked response to at least one stimulation pulsedelivered at a first energy level; and concurrently increasing both theamplitude and duration components of the controlled energy level inresponse to the loss-of-capture criteria until a capture criteria ismet.
 18. The method of claim 17 additionally comprising the step ofperiodically determining the set of characteristics for each energylevel.
 19. The method of claim 17 additionally comprising the step ofpredetermining the set of characteristics for each energy level.
 20. Themethod of claim 17 additionally comprising the step of remotelyprogramming the set of characteristics for each energy level.
 21. Themethod of claim 17 wherein the step of increasing the amplitude andduration components of the controlled energy level is performedaccording to an essentially linear relationship specified by theequation: amplitude=(slope*duration)+offset where slope and offset areconstant values.
 22. The method of claim 21 additionally comprising thestep of setting the slope to a value between 0.3 and 3.0volts/millisecond.
 23. The method of claim 22 additionally comprisingthe steps of: setting the slope to a value of essentially 1.0volts/millisecond; and setting the offset to a value that is a functionof a determined strength-duration curve characterized by a chronaxie anda rheobase.
 24. The method of claim 23 additionally comprising the stepof setting the offset to a value defined by the equation:offset=(2*rheobase)−chronaxie where rheobase is in volts and chronaxieis in milliseconds.
 25. The method of claim 23 additionally comprisingthe step of setting the offset to a value defined by the equation:offset=rheobase.
 26. The method of claim 23 additionally comprising thestep of periodically determining the chronaxie and rheobase.
 27. Themethod of claim 17 additionally comprising the step of storing within atable a plurality of energy levels defined by amplitude and durationcomponents.
 28. The method of claim 27 additionally comprising the stepof storing within the table at least one pair of sequential energylevels where the amplitude component is essentially the same between thesequential energy levels and the duration component increases betweenthe sequential energy levels.
 29. The method of claim 27 additionallycomprising the step of storing within the table include at least onepair of sequential energy levels where the amplitude component increasesbetween sequential energy levels and the duration component decreasesbetween sequential energy levels.
 30. The method of claim 23additionally comprising the steps of: determining a starting controlledenergy level having an amplitude component and a duration component inresponse to the determined strength-duration curve; and determining aninitial controlled energy level for delivery by the pulse generator byincreasing both the amplitude and duration components of the startingcontrolled energy level to achieve a safety margin from the determinedstrength-duration curve.
 31. The method of claim 30 additionallycomprising the step of setting the initial controlled energy level to avalue greater than the starting controlled energy level by a variablesafety margin.
 32. An implantable stimulation device configured forstimulating muscle tissue through at least one electrode implanted inelectrical contact with selected muscle tissue, the stimulation devicecomprising: a pulse generator configured for electrical coupling to theelectrode and configured to generate stimulation pulses at a controlledenergy level to thereby stimulate the muscle tissue, wherein thecontrolled energy level is defined by a set of characteristics includingan amplitude component and a duration component; a detection circuitconfigured for electrical coupling to the electrode and configured toreceive signals for determining the presence or absence of an evokedresponse to each of the stimulation pulses; a controller, coupled to thepulse generator and the detection circuit, for increasing the controlledenergy level from a first energy level to a second energy level inresponse to a loss-of-capture criteria related to the absence of anevoked response to at least one stimulation pulse generated at the firstenergy level; and wherein the controlled energy level is increasedaccording to a relationship between the amplitude and durationcomponents that is predominantly defined by an essentially linearrelationship specified by the equation:amplitude=(slope*duration)+offset  where slope and offset are constantvalues.
 33. The implantable stimulation device of claim 32 wherein theset of characteristics for each energy level is determined by thecontroller.
 34. The implantable stimulation device of claim 32 whereinthe set of characteristics for each energy level is predetermined. 35.The implantable stimulation device of claim 32 wherein the set ofcharacteristics for each energy level is remotely programmable.
 36. Theimplantable stimulation device of claim 32 wherein the slope is between0.3 and 3.0 volts/millisecond.
 37. The implantable stimulation device ofclaim 36 wherein the slope is essentially 1.0 volts/millisecond and theoffset is a function of a strength-duration curve characterized by achronaxie and a rheobase.
 38. The implantable stimulation device ofclaim 37 wherein the offset is defined by the equation:offset=(2*rheobase)−chronaxie where rheobase is in volts and chronaxieis in milliseconds.
 39. The implantable stimulation device of claim 37wherein the offset is defined by the equation: offset=rheobase.
 40. Theimplantable stimulation device of claim 37 wherein the controllerperiodically determines the chronaxie and rheobase.
 41. The implantablestimulation device of claim 32 additionally comprising a table fordefining a plurality of energy levels defined by amplitude and durationcomponents.
 42. The implantable stimulation device of claim 41 whereinthe energy levels defined within the table include at least one pair ofsequential energy levels where the amplitude component is essentiallythe same between the sequential energy levels and the duration componentincreases between the sequential energy levels.
 43. The implantablestimulation device of claim 41 wherein the sets of energy levels definedwithin the table include at least one pair of sequential energy levelswhere the amplitude component increases between sequential energy levelsand the duration component decreases between sequential energy levels.44. The implantable stimulation device of claim 40 wherein a startingcontrolled energy level having an amplitude component and a durationcomponent is determined in response to the strength-duration curve andan initial controlled energy level for delivery by the pulse generatoris determined by increasing both amplitude and duration components ofthe starting controlled energy level to achieve a safety margin from thedetermined strength-duration curve.
 45. The implantable stimulationdevice of claim 44 wherein the initial controlled energy level isgreater than the starting controlled energy level by a variable safetymargin.
 46. The implantable stimulation device of claim 44 wherein thestarting controlled energy level has an amplitude component of twice therheobase and a duration component of the chronaxie.
 47. The implantablestimulation device of claim 44 wherein the starting controlled energylevel corresponds to the intersection between the strength-durationcurve and the essentially linear relationship defining the controlledenergy level.
 48. An implantable stimulation device for stimulating apatient's body tissue, the stimulation device comprising: means forperiodically delivering a stimulation pulse to the patient's bodytissue, the stimulation pulse having a controlled energy level, whereinthe controlled energy level is defined by a set of characteristicsincluding an amplitude component and a duration component; means fordetecting the presence or absence of an evoked response generated by thepatient's body tissue in response to the stimulation pulse; and meansfor determining whether a loss-of-capture criteria is satisfied inresponse to the absence of an evoked response to at least onestimulation pulse; means for increasing the controlled energy level whenthe loss-of-capture criteria is satisfied according to a relationshipbetween the amplitude and duration components that is predominantlydefined by an essentially linear relationship specified by the equation:amplitude=(slope*duration)+offset where slope and offset are constantvalues.
 49. An implantable cardiac stimulation device comprising: pulsegenerator means for providing stimulation pulses to cardiac tissue, suchstimulation pulses having selectable pulse shapes, each pulse beingdefined by a pulse amplitude and pulse width; detection means fordetecting the presence or absence of an evoked response corresponding toeach stimulation pulse and indicative thereof of the presence or absenceof capture; control means for controlling the stimulation pulseamplitude and pulse width in response to a loss of capture criteria; andwherein the control means simultaneously increases both the pulseamplitude and pulse width until a capture criteria is met.
 50. Thestimulation device of claim 49 wherein the pulse amplitude and pulsewidth are related by the equation: pulse amplitude=slope*pulsewidth+offset wherein slope and offset are constant values.
 51. Thestimulation device of claim 50 wherein the slope is in the range of 0.3to 3.0 volts/millisecond.
 52. The stimulation device of claim 50 whereinthe control means includes means for determining the chronaxie andrheobase of the cardiac tissue.
 53. The stimulation device of claim 52wherein the value of the offset is defined by the equation:offset=(2*rheobase)−chronaxie where rheobase is in volts and chronaxieis in milliseconds.
 54. The stimulation device of claim 52 wherein thevalue of the offset is defined by the equation: offset=rheobase.